Physics-based computational modeling of subsurface CO₂ migration constitutes the primary tool to assess geologic carbon storage. Such models are often required to plan injection operations and assess hazards such as CO₂ migration into units above the storage formation. Here, we present three tools developed to increase fidelity of black-oil type geologic carbon storage models in the open-source MATLAB Reservoir Simulation Toolbox. These tools, which are published in the $\mathtt{\text{co2lab-mit}}$ module, include functionality to: (1) Calculate and output PVT properties of miscible brine and CO₂ as a function of pressure, temperature, and salinity; (2) account for relative permeability hysteresis, necessary to model residual trapping; and (3) model CO₂ transport due to concentration gradients (molecular diffusion). We validate our implementation with published results including experimental observations, present $\mathtt{\text{MRST}}$ examples, and conclude with some remarks on applicability, limitations, and potential extensions. Source code and examples are provided.